In using radar for fire control and missile weapon launch purposes, detection and tracking of airborne targets in the presence of surface clutter is required. This surface clutter constitutes the radar reflections from the surface of the earth which constitutes a part of the video return through either the main beam of the antenna or its side lobes; this return (known as the altitude line) is often so strong that weaker reflections from smaller airborne targets are completely masked.
Pulse doppler radars have been developed which solve a portion of this problem. They overcome the serious masking by recognizing the doppler effect on the radar return caused by the variation of the relative radial velocity between the aircraft and the target and the aircraft and the earth surface.
Thus the reflections have specific frequencies that may be separated out by means of a comb filter or other suitable detector coupled to the radar return analysis circuitry. However, this technique does not solve the entire problem. In particular, the mode of operation wherein automatic acquisition and tracking of the airborne target is to be provided, cannot be accomplished with this system without prior knowledge of the specific target doppler frequency at the moment of acquisition. Without this knowledge, the sweeping range gate would ignore the target. If, on the other hand, a pulse radar attempts to automatically acquire a target, the range gate will lock up the first video return that it encounters as it sweeps out in range and thus often locks onto a false information return. It is therefore the object of this invention to provide a radar with an inherent ability to recognize and discriminate between an airborne target and the surface clutter whether the clutter is masking the target or is picked up by the antenna side lobes. It is also the object of this present invention to provide discrimination during an automatic acquisition mode wherein the range gate automatically sweeps out in range and locks onto the airborne target.
The present invention puts to use the amplitude modulation appearing on the target video return during the period of time the conical scan broadcast by the radar antenna dwells on the airborne target. This modulation results from the positioning of the target relative to the movement of the conical scan sometimes referred to as "nutation of the feed horn" as it illuminates the target. Systems are known wherein the conical scan feature is employed in angle tracking circuits to cause the angle servos to drive the antenna in an appropriate direction to place the target in the center of the nutating beam. Examplary circuits are disclosed in the following U.S. Pat. Nos.: Slusser 2,776,422; Sommers 3,082,415; McCoy 3,206,753; Schmutz et al 3,231,759; Adams, 3,307,183; Jones et al 3,445,663; and Waters 3,588,989. For example, the U.S. Pat. No. 3,307,183 to Adams discloses a conical scan radar system which may be operated either in the pulse or CW modes; the purpose of the invention is to provide a means for avoiding jamming by external sources. The amplitude modulated return is used for range tracking by comparison against a reference sine wave in order to arrive at a correction signal to center the target in the center of the conical scanning beam. But no disclosure is made in this patent of use of the amplitude modulated return to discriminate between the target video information and ground clutter return. U.S. Pat. No. 3,231,759 to Schmutz et al discloses a pulse radar system wherein the received signal is amplitude modulated unless the target is on the axis of symetry of the transmitted beam. In this circuit, a control voltage is generated from the modulated echo for operating the automatic tracking system and again centering the target. U.S. Pat. No. 3,083,415 to Sommers discloses a target acquisition system wherein the conically scanned beam acts in conjunction with a gate which locks the radar onto the target as bearing elevation and range vary. None of these patents or the other patents cited above discloses use of the amplitude modulated return signal for discrimination of target information from ground clutter to enable a track gate to identify and lock on to the target. This invention has the unique ability not disclosed in the prior art of utilizing the amplitude modulated return information to discriminate between an airborne target and surface clutter, to thereby discover targets flying low against the surface of the earth and enable the gated circuitry to initially lock onto and remain onto the target to be tracked.